Medium- and heavy-duty trucks, motor coaches, and transit buses - collectively, "medium- and heavy-duty vehicles", or MHDVs - are used in every sector of the economy. The fuel consumption and greenhouse gas emissions of MHDVs have become a focus of legislative and regulatory action in the past few years. Reducing the Fuel Consumption and Greenhouse Gas Emissions of Medium- and Heavy-Duty Vehicles, Phase Two is a follow-on to the National Research Council's 2010 report, Technologies and Approaches to Reducing the Fuel Consumption of Medium-and Heavy-Duty Vehicles. That report provided a series of findings and recommendations on the development of regulations for reducing fuel consumption of MHDVs. This report comprises the first periodic, five-year follow-on to the 2010 report. Reducing the Fuel Consumption and Greenhouse Gas Emissions of Medium- and Heavy-Duty Vehicles, Phase Two reviews NHTSA fuel consumption regulations and considers the technological, market and regulatory factors that may be of relevance to a revised and updated regulatory regime taking effect for model years 2019-2022. The report analyzes and provides options for improvements to the certification and compliance procedures for medium- and heavy-duty vehicles; reviews an updated analysis of the makeup and characterization of the medium- and heavy-duty truck fleet; examines the barriers to and the potential applications of natural gas in class 2b through class 8 vehicles; and addresses uncertainties and performs sensitivity analyses for the fuel consumption and cost/benefit estimates.